Eight compounds, exhibiting anti-HIV activity, which contain the O-alkyl alkoxime ether moiety were examined by electron-ionization mass spectrometry (EIMS) and H-nuclear magnetic resonance (1H-NMR). Each compound was assigned a syn-configuration from the chemical shift of the "aldehydic" proton of the aldoxime in a freshly prepared solution. Syn-anti photoisomerization of the 0-alkyl aldoxime ether moiety of these compounds occurred rapidly in solutions exposed to light. The photostationary state was typically attained in less than one hour when samples were exposed to direct sunlight, and was composed of approximately equal proportions of the syn- and anti- isomers. The isomers were separable by chromatographic methods (gas chromatography or liquid chromatography) and yielded essentially identical EI or thermospray mass spectra. 1H-NMR of the photoisomerized reaction mixture was used to assign the configuration of the aldoxime isomers. With careful exclusion of all light, syn-anti isomerization was not observed. Photoisomerization was observed in a variety of solvents. The rate at which the photostationary state was attained and the final syn/anti ratio were relatively independent of solvent and whether or not the solution was degassed prior to irradiation. The results of these experiments suggest that the mechanism of isomerization is unimolecular and not the result of a free radical process.